The water supercooled regime as described by four common water models

The interplay between dynamic heterogeneities and structure of bulk liquid water: A molecular dynamics simulation study

In order to study the interplay between dynamical heterogeneities and structural properties of bulk liquid water in the temperature range 130-350 K, thus including the supercooled regime, we use the explicit trend of the distribution functions of some molecular properties, namely, the rotational relaxation constants, the atomic mean-square displacements, the relaxation of the cross correlation functions between the linear and squared displacements of H and O atoms of each molecule, the tetrahedral order parameter q and, finally, the number of nearest neighbors (NNs) and of hydrogen bonds (HBs) per molecule. Two different potentials are considered: TIP4P-Ew and a model developed in this laboratory for the study of nanoconfined water. The results are similar for the dynamical properties, but are markedly different for the structural characteristics. In particular, for temperatures higher than that of the dynamic crossover between "fragile" (at higher temperatures) and "strong" (at lower temperatures) liquid behaviors detected around 207 K, the rotational relaxation of supercooled water appears to be remarkably homogeneous. However, the structural parameters (number of NNs and of HBs, as well as q) do not show homogeneous distributions, and these distributions are different for the two water models. Another dynamic crossover between "fragile" (at lower temperatures) and "strong" (at higher temperatures) liquid behaviors, corresponding to the one found experimentally at T(∗) ∼ 315 ± 5 K, was spotted at T(∗) ∼ 283 K and T(∗) ∼ 276 K for the TIP4P-Ew and the model developed in this laboratory, respectively. It was detected from the trend of Arrhenius plots of dynamic quantities and from the onset of a further heterogeneity in the rotational relaxation. To our best knowledge, it is the first time that this dynamical crossover is detected in computer simulations of bulk water. On the basis of the simulation results, the possible mechanisms of the two crossovers at molecular level are discussed.

In-layer stacking competition during ice growth

When ice grows, the growth rates are unequal [corrected] along different growth directions and some layers contain planar defective regions. With the aim of helping to understand these phenomena, we report the molecular dynamics simulations of ice growth on the basal and prismatic faces of initial hexagonal ice, using the TIP5P-E water model. By presenting the time evolution of the two-dimensional density profiles of water molecules in each layer and the kinetics of layer formation during ice growth at the temperature of 11 K supercooling, we show that two forms of ice arrangements, hexagonal and cubic, develop competitively within the same ice layer on the basal face, whereas such in-layer stacking-competition is insignificant on the prismatic face. It is shown that, on the basal face, the occurrence of significant in-layer stacking competition in one of the layers significantly delays the layer formation in several overlying layers and explains the overall delay in ice growth on the basal face compared to that on the prismatic face. In addition, it is observed that large planar defects form on the basal face, as a consequence of the long-lasting in-layer stacking competition when the overlying layer grows rapidly.